some things to consider when chosing a game engine are...

1) target audience
2) type of game
3) talent of design team
4) technology requirements of the game

Could A6 professional be used for a professional title, sure, but it's not the best choice for all types of games.

Why would I not use A6 professional... If I wanted to create a game featuring the latest eye candy and graphics technology comparable to a AAA title, A6 is not the answer. Also, I've yet to be convinced that A6 is an option for multi-player gaming.